Many road signs are designed to break if you crash into them

A
slip base built by Traffic Safety Products, a division of Eberl
Iron Works, Inc.Elizabeth
Durand

When a car hits a sign post and knocks it to the ground, most of
us assume the car was going fast enough to completely uproot
the pole from the ground.

But falling down is exactly what many posts are made to do
under impact. In fact, they’re designed to break in two.

As explained in a
blog post for the podcast 99% Invisible, many roadside
posts actually use a piece of hardware called a slip base, which
allows the pole to break in a way that helps save
drivers and reduce damage to cars during an accident.

Posts that use slip bases are split into two pieces, joined
together just above the ground.

The upper post holds the sign or electrical wire, and has a plate
at its base. The bottom piece, which is installed in the
ground, has a plate on its top. The two plates are joined using
bolts — but those bolts are designed to give way
at certain threshold of impact.

When a car slams into the post, the movement of the plates
cause the bolts to break or dislodge, thereby disconnecting the
two pieces, as seen in this retro demo video:

The bottom piece stays in the ground, allowing the car to
drive over it, while the top one disconnects. If a sign is held
up by two poles, the slip base also allows a car to knock one
post out of the way and keep driving, rather than getting lodged
underneath or stuck between the two. Either way, the
collision isn't as abrupt, and the impact on the car and its
passengers is less fatal. Task
Force 13

Though this system might sound dangerous — since it seems
like the post could simply fall onto the car— the slip base
system is designed with torque that causes the top
piece of a post to get knocked out of the way, rather than
falling right onto the car.

Some slip bases are even built with inclines to accommodate the
anticipated direction of impact (on a one-way street or highway,
for example).

So the next time you walk by a sign post on the sidewalk,
look down — you’ll likely see some kind of slip base system. The
main clue is a set of bolts that are clearly visible above the
ground, silently protecting your life.